Patent Document 1 describes a hydraulic drive system as below. A first holding valve is provided in an actuator line between the bottom-side chamber of a boom cylinder and a directional control valve (a changeover valve). A recovery pump motor is disposed via a second holding valve in a line branching from the actuator line. The recovery pump motor is connected on the discharge side thereof to a tank via a proportional restrictor. This hydraulic drive system is such that during the boom-lowering operation in midair in which the boom cylinder can be contracted under the self-weight of a front work implement, the recovery pump motor is rotated by opening the second holding valve to discharge the hydraulic fluid from the bottom-side chamber of the boom cylinder. The rotation of the recovery pump rotates a generator to recover the positional energy of the front work implement. If the front work implement is brought into contact with the ground for excavating, a directional control valve is switched so as to supply hydraulic fluid from a hydraulic pump to the rod-side chamber of the boom cylinder. In addition, the first and second holding valves are opened to discharge the hydraulic fluid in the bottom-side chamber of the boom cylinder for ensuring a necessary excavating force.
Patent Document 2 describes a hydraulic drive system that includes a jack-up changeover valve and a flow control valve. The jack-up changeover valve is switched when the pressure in the bottom-side chamber of a boom cylinder becomes equal to or higher than a predetermined pressure. With the switching operation of this changeover valve the flow control valve opens or closes a line adapted to supply hydraulic fluid from a main pump to the rod-side chamber of the boom cylinder. The hydraulic drive system is such that during the boom-lowering operation in midair in which the boom cylinder can be contracted under the self-weight of a front work implement, the jack-up changeover valve is switched to close the flow control valve. The supply of the hydraulic fluid from the main pump to the rod-side chamber of the boom cylinder is blocked. In addition, the hydraulic fluid discharged from the bottom-side chamber of the boom cylinder is supplied to the rod-side chamber for recovery. Thus, pump-consumption horsepower is controlled during the boom-midair lowering operation. During the jack-up in which the boom cannot be lowered under self-weight, the jack-up changeover valve is not switched because of the low pressure in the bottom-side chamber of the boom cylinder. The flow control valve is held at an open position and hydraulic fluid is supplied from the main pump to the rod-side chamber of the boom cylinder. Thus, the jack-up operation is enabled.